Activation Synthesis Theory: The idea that during sleep, an automatic activation system in the brain produces a series of random electrical discharges that the sleeper roughly ties together by creating a storyline. An opposing theory to the traditional Freudian idea that dreams are secrets, fears, and the like, rising from the unconscious.

Circadian Rhythm: Consistent pattern of cyclical body activities that lasts approximately 24 hours; also known as the biological clock.

Functionalism: The perspective on mind and behavior that focuses on the examination of the organism's interactions with its environment. The study of the contents of consciousness; associated with William James.

Hypnagogic State: An alternate state of consciousness at the onset of sleep, and the perceptions, fantasies, and energy levels provoked by that state.

Lucid Dreaming: The state of being consciously aware, while sleeping, that you are dreaming.

Structuralism: The view that all human mental experience can be understood as the combination of simple events or elements, and that the underlying structure of the human mind can be revealed by analyzing all basic elements of sensation. The study of the how and why of experience; associated with Wilhem Wundt.